Inexpensive integrated circuits often rely on a plastic leadframe form of packaging that is adapted for mounting on printed circuit boards via surface mounting technology rather than utilizing pins that must be inserted into holes in the printed circuit board. This type of integrated circuit typically has a die that is mounted on a leadframe. The die and part of the leadframe are encapsulated in an epoxy resin. The portions of the leadframe that extend outside the encapsulation provide electrical connections. These portions are bent into configurations that allow the package to be mounted onto the printed circuit board via surface mounting techniques.
This type of packaging arrangement is well suited for integrated circuits that do not dissipate significant amounts of heat. However, in applications in which the required heat dissipation is high, this packaging arrangement has problems. The heat from the small integrated circuit die must be transferred to some larger surface that can, in turn, transfer the heat to the air surrounding the heat-dissipating surface. There are two types of heat dissipating arrangements. In the first, the heat is transferred to an individual heat radiating structure that is coupled to the individual package. This structure can include fins to increase the surface area, and hence, provide sufficient surface area to transfer the heat to the surrounding air. While such packages can dissipate a significant amount of heat, the cost of the individual radiators is often prohibitive.
A second scheme utilizes the core of the printed circuit board to remove the heat from the various integrated circuits mounted on the board. The core of the printed circuit board has a much larger area than the area of the dies in the integrated circuits. In addition, some form of active heat transfer arrangement can be included on the printed circuit board or in the enclosure containing the printed circuit board. This heat dissipation scheme requires that the heat be moved from each integrated circuit die to the printed circuit board. One or more of the leads that extend from the package are often used for this heat removal function. For example, a lead that is connected to ground in the package can provide a heat path to the core of the printed circuit board, which typically, is also connected to ground. Unfortunately, these leads tend to have a small cross-sectional area and a relatively long length. Hence, the amount of heat that can be moved is limited. In addition, the thermal mass of the lead frame area is also limited, and hence, the package is subject to large thermal fluctuations as the integrated circuit is turned on and off.
Packages in which the die is mounted on a heat sink within the package and in which the heat sink is directly connected to the printed circuit board are also known. If the heat sink forms the bottom surface of the package, then the heat can be directly transferred to the printed circuit board. Unfortunately, such arrangements are subject to packaging failures since the heat sink is not encapsulated on all sides by the packaging material, and hence, can separate from the packaging material during repeated heating cycles during the operation of the packaged integrated circuit.
Other arrangements in which the heat sink is separated from the bottom of the package to allow encapsulation have also been proposed. In these arrangements, the heat sink is directly connected to the surface below the package by vias that are filled with solder. Such packages are significantly more expensive than the simple leadframe package, and hence, are not well suited for low cost integrated circuits such as LEDs.